1. Field of the Invention
The present invention relates to strontium nitrate and a method for manufacturing the same, and more particularly is aimed at providing strontium nitrate that has a low impurity content and that is suitable (as an oxidant) for gas generation in automotive airbags.
2. Description of the Related Art
Strontium nitrate emits red light when ignited, and is thus predominantly used as a material for smoke candles. Strontium nitrate is used in small amounts as a lighting material for briquettes, as well as an intermediate starting material for semiconductors, fluorescents, and the like.
In another recent application, the compound* has come to be used as an oxidant for gas generation in automotive airbags due to this compound's low deliquescence or hygroscopicity, relative stability against heat, and the like. Metal oxidizers have been used in the past as oxidants for the gas-generating agents of airbags, but these oxidizers cannot necessarily be considered as having adequate deliquescence, hygroscopicity, stability against heat, or the like, and a transition to strontium nitrate has gradually come about, as described above. The following methods can be cited as examples of methods for manufacturing such strontium nitrate.
Methods for manufacturing strontium nitrate after strontium carbonate is first manufactured as a starting material include methods in which strontium carbonate is manufactured by reductive roasting using celestite (principal component: SrSO.sub.4) as a starting material EQU SrSO.sub.4 +2C.fwdarw.SrS+2CO.sub.3 EQU SrS+CO.sub.2 +H.sub.2 O.fwdarw.SrCO.sub.3 +H.sub.2 S,
and methods for manufacturing strontium carbonate by the soda process EQU SrSO.sub.4 +Na.sub.2 CO.sub.3.fwdarw.SrCO.sub.3 +Na.sub.2 SO.sub.4.
The strontium carbonate (SrCO.sub.3) obtained by these methods is dissolved in dilute nitric acid, impurities are removed from the resulting strontium nitrate solution, and the product is then concentrated, cooled, and crystallized until a crystal film forms, yielding strontium nitrate tetrahydrate (Sr(NO.sub.3).sub.2.4H.sub.2 O).
It is commonly known that an anhydrous salt can be obtained when a strontium nitrate solution from which impurities have been removed is concentrated in an evaporator, crystals are allowed to precipitate under heat, the precipitate is centrifuged while heated, and the product is dried at 100.degree. C. or higher. EQU SrCO.sub.3 +2HNO.sub.3.fwdarw.Sr(NO.sub.3).sub.2 +CO.sub.2 +H.sub.2 O
(Kagaku Binran [Chemistry Handbook], Oyo Kagaku-hen, 1986)
Japanese Unexamined Patent Application 57-209820 discloses a method for obtaining strontium nitrate containing 0.89 wt % Ca, 0.44 wt % Ba, 6.6 ppm Mg, 37 ppm SiO.sub.2, 0.43 ppm Na, 5 ppm Al, and no more than 0.1 ppm Fe by a technique in which the natural mineral celestite (containing strontium sulfate as its principal component) is mixed, heated, and reacted with crystal water-containing calcium nitrate; the reaction mixture is then brought into contact with water; the strontium nitrate of the reaction product and the unreacted calcium nitrate are extracted with water; the extract is heated to evaporate off the water; crystals of strontium nitrate are allowed to precipitate; the precipitate is separated from the mother liquor; and the product is washed with water and dried.
In addition, a method for removing impurities from the resulting strontium nitrate is described, for example, in Japanese Examined Patent Application 58-35935, which discloses a method in which heavy metals or magnesium are/is precipitated and separated out as hydroxides by adding calcium hydroxide to a raw strontium nitrate solution containing heavy metals or magnesium, and the pH is adjusted to 7-8.
According to Japanese Examined Patent Application 58-35935, celestite (SrSO.sub.4), which is a naturally occurring strontium sulfate, is used as a starting material; strontium carbonate precipitated by the double decomposition technique EQU SrSO.sub.4 +Na.sub.2 CO.sub.3.fwdarw.Na.sub.2 SO.sub.4 +SrCO.sub.3
is separated from the reaction mixture and washed; and an adhered sodium sulfate solution is removed, yielding a cake of raw strontium carbonate.
Typically, such raw strontium carbonate has the following dry composition: 86% SrCO.sub.3, 8% CaCO.sub.3, 2% MgCO.sub.3, 2% Na.sub.2 SO.sub.4, 1.5% inert matter, 1% BaCO.sub.3, 0.5% BaSO.sub.4, and 0.1% Na.sub.2 CO.sub.3. A raw strontium carbonate cake is subsequently made into a slurry having a solids content (dry basis) of about 0.5-6%; nitric acid with a concentration of, for example, 69% is added; the slurry* is allowed to react with the nitric acid at about 40-90.degree. C.; nitric acid is added until carbon dioxide stops forming; excess acidity is then corrected by adding a small amount of a moistened cake of strontium carbonate; and the pH is adjusted to about 3-4.
Calcium hydroxide is subsequently added to adjust the pH to about 7-8 and to precipitate the heavy metal or magnesium hydroxides present in the nitrate solution. The precipitate is filtered at 40-100.degree. C., yielding a raw strontium nitrate solution. Typically, the resulting raw strontium nitrate solution has the following composition: 39% Sr(NO.sub.3).sub.2, 6% Ca(NO.sub.3).sub.2, 1% NaNO.sub.3, 1% Mg(NO.sub.3).sub.2, and 0.4% Ba(NO.sub.3).sub.2, with water constituting the balance.
This raw strontium nitrate solution is crystallized by vaporization at 60-90.degree. C., yielding solid strontium nitrate. The product* is separated and washed, and the adhered mother liquor is removed and dried. Typically, the anhydrous strontium nitrate crystals thus obtained have the following composition: 98% Sr(NO.sub.3).sub.2, 1.0% Ba(NO.sub.3).sub.2, 0.35% Ca(NO.sub.3).sub.2, 350 ppm NaNO.sub.3, and 550 ppm Mg(NO.sub.3).sub.2.
Japanese Unexamined Patent Application 9-77516 discloses another method for removing impurities. In this application, the following method is described as an intermediate step of a method for manufacturing high-purity strontium carbonate: nitric acid is added to a strontium nitrate-containing aqueous solution (starting material) to precipitate the strontium nitrate, and the precipitated strontium nitrate is separate into a liquid and solid fractions, yielding strontium nitrate with a low content of Na, K, Fe, Ni, U, Th, Ca, and other impurities.
The following impurities are contained in this strontium nitrate starting material: 270 ppm Ca, 95 ppm Na, 12 ppm K, 1 ppm Fe, 1 ppm Ni, 0.1 ppb U, and 0.1 ppb Th. Nitric acid with an equivalent acid concentration of 5.3 N is added to this aqueous solution, the solution* is first heated to 80.degree. C. and then cooled to room temperature, and the precipitated strontium nitrate is separated into a solid and liquid fractions. The following impurities are contained in the recrystallized strontium nitrate: 4 ppm Ca, 3 ppm Na, less than 1 ppm K, less than 0.1 ppm Fe, less than 0.1 ppm Ni, less than 0.03 ppb U, and less than 0.03 ppb Th.
In addition, the following method for purifying strontium nitrate is disclosed in U.S. Pat. No. 3,065,052: an aqueous solution of strontium nitrate containing barium impurities is prepared, this solution is gradually adjusted to at least the neutral range with nitric acid, the adjusted solution is mixed with a chromic acid solution to produce a strongly acidic solution, a strontium hydroxide solution is gradually added to the resulting solution containing a nitrate and an acid to adjust the pH to 11 while this solution is agitated, this pH is kept unchanged for several minutes to allow the barium impurities to precipitate as barium chromate, the solution is then filtered to remove the barium chromate, nitric acid is subsequently added to the filtrate to gradually bring the pH to about 1 while the filtrate is boiled, and the filtrate is then evaporated to precipitate strontium nitrate crystals, yielding strontium nitrate having the following average composition: 99.85% Sr(NO.sub.3).sub.2, 0.05% Ba(NO.sub.3).sub.2, 0.004% NaNO.sub.3, Ca(NO.sub.3).sub.2 (undetected), and the like.
In addition, the following quality is stipulated by JIS for anhydrous strontium nitrate reagents, in terms of metal elements: no more than 0.0005 wt % heavy metals (as Pb), no more than 0.0005 wt % Fe, no more than 0.02 wt % Ca, no more than 0.01 wt % K, and no more than 0.02 wt % Na.
The above-described conventional strontium nitrate contains large amounts of Ba, Ca, and Na. It is thus impossible to conclude that adequate deliquescence or hygroscopicity can be achieved when such conventional strontium nitrate is used as an oxidant for airbags. The explosive characteristics of gas generation in airbags will therefore be unstable if the airbags are stored for a long time under adverse conditions, absorbing moisture. In addition, the presence of Ba in strontium nitrate makes it likely that barium will remain as an oxide following gas generation in an airbag.
Another concern is that Cr and other impurities will be dispersed in the environment after gas generation in an airbag, rendering manufacturing methods based on conventional chromic acid treatments disadvantageous in that Cr will remain as an impurity in strontium nitrate.
In addition, methods aimed at removing barium as chromates cause contamination of strontium nitrate with chromium and require that wastewater treatment be performed to remove chromic acid, a compound whose use is subject to strict regulations.
In addition, starting materials often contain NH.sub.3, and traces of NH.sub.3 (for example, about 10 wt ppm) are sometimes contained in the final product. Particle diameters are poorly controlled. pH values of water solutions are nonuniform because of inadequate removal of mother liquors during purification. In addition, high moisture content sometimes results.
Traces of NH.sub.3 may therefore be present in strontium nitrate; the mean particle size of a powder may be too small, or there may be too many fine particles; and the pH of a product dissolved in water may be too high or too low. Thus deliquescence and hygroscopicity may become more pronounced, and reactivity might increase and poor stability result, making it more difficult to manufacture airbags, inflators, or the like. Other drawbacks include poor thermal stability or other characteristics of the final product. In addition, explosion characteristics are sometimes inadequate when the moisture content is high.
A strong need therefore existed for developing a method for manufacturing high-purity strontium nitrate which is largely devoid of the above-described drawbacks, in which the content of barium, sodium, calcium, chromium, and ammonia is reduced, whose particle size and pH are controlled, which has a low moisture content, and which is suitable for use as an oxidant for automotive airbags or the like; and for manufacturing strontium nitrate for which no chromic acid wastewater treatments are required.